Record-hot August, dry year leave high fire risk in county

A hot summer and a dry year have left parched hillsides in the so-called "urban-wildlands" interface this year.

A hot summer and a dry year have left parched hillsides in the so-called "urban-wildlands" interface this year. (Robert Krier San Diego Union-Tribune)

Robert Krier

This month will go down as San Diego’s hottest August on record. It was the second hottest month, any time of year, in city history. The month set or tied 15 daily records for either the highest maximum or minimum temperature.

The August heat follows a similarly toasty July, making the July/August combination the second hottest in city history. And the heat comes near the tail end of the second driest rainfall season on record, dating back to 1850.

The heat and the prolonged dryness set up the county for a challenging fall, which historically is when the region’s biggest and most destructive wildfires have struck.

“The whole area has had a very hot summer,” said Matthew Moreland, the meteorologist in charge of the National Weather Service’s Rancho Bernardo office. “We’re very concerned about the fire situation.”

Statewide, 2018 has already been a horrendous year for fires. In Cal Fire’s jurisdiction, the number of fire starts this year is up slightly, from 4,170 in 2017 to 4,434 in 2018. But the acreage burned has skyrocketed from 228,803 in 2017 to to 876,428, as of Monday.

The Mendocino Complex fire, which has affected four Northern California counties and is still not fully contained, has burned more than 410,000 acres and is the largest fire in state history. The Carr fire in Shasta and Trinity counties is the seventh largest in state history and has burned another 229,000 acres.

“It paints a stark picture, but that’s what we’ve been dealing with all year,” Cal Fire Capt. Isaac Sanchez said. “We’re prepared for the worst.”

San Diego's hottest Augusts

1. 2018 78.2* degrees (average temperature)

2. 1983 77.4

3. 1984 76.6

4. 1981 75.8

5. 1971 75.4

Source: National Weather Service

* - Includes temperatures forecast for Friday, Aug. 31.

The severity of the fire season could depend largely on upcoming weather conditions: the strength, frequency and timing of Santa Ana winds, plus the arrival and performance of the predicted El Niño phenomenon.

But just how bad are the conditions in the county’s back country as the peak fire season approaches? Are they as bad as they were before the fall of 2003 and 2007, among the worst fire seasons in county history?

“It’s a mixed signal,” said Rob Krohn, a meteorologist with the U.S. Forestry Service’s Predictive Services Branch in Riverside. “There are things you could point to to say it’s worse, and there are thing you could point to to say it’s slightly better.”

In 2003, there was much more live vegetation out there to burn, Krohn said. This year, he said, plants are smaller and there are fewer per acre because of the five-year drought earlier this decade. And there is also less of a grass crop this year because of the scant rains last winter and spring.

One of the key measures of fire potential the experts weigh is the moisture content of both the live and dead vegetation. The less moisture present, the more readily the material will burn.

In late August, the so-called “dead-fuel” moisture levels in San Diego County are not quite at record levels, but they are still very low — below 10 percent of the historical average, Krohn said.

“It’s drier than it was in 2003, but probably not as dry as 2007,” Krohn said. “It’s probably splitting hairs. And there’s still a lot of dead vegetation out there.”

San Diego County is in marginally better shape than counties to the north because of the county’s fairly active monsoon season this summer. But the areas where the storms hit were in the eastern part of county, far from population centers.

“Unfortunately, the driest areas are in the urban-wildland interface,” where homes butt up against undeveloped canyons in the central part of the county, Krohn said.

Even where monsoon rains did drench a region (spots near Mount Laguna recorded more than 3 inches one day this month), the reprieve from high fire danger is usually temporary. Most of the rain from a strong thunderstorm runs off and doesn’t soak into the ground, so trees and plants can’t use the moisture.

“It’s good, but it’s not the most efficient rain there is,” Krohn said.

Weather’s big role

The status of the back country vegetation is a key component of the region’s fire risk, but weather patterns in the fall can have an even bigger impact. At the moment, the weather conditions that evolved into a perfect firestorm late last year appear less likely this year, Krohn said. But that doesn’t mean the fall will be devoid of extreme risk.

It comes down to the prevalence and performance of Santa Anas. Long-range forecast models project an average to slightly below average number of Santa Anas, at least through November. Last year, the Santa Anas were frequent, long-lasting and strong — just as the models had predicted.

“We’re not expecting a repeat of last year, by any means,” Krohn said. “That being said, we’ll probably have a couple of (Santa Ana winds) before we get wetting rains.”

The timing of those first good rains will be crucial. Last winter, San Diego remained bone dry all the way through December. Only 0.09 of an inch of rain fell from Oct. 1 through Dec. 31; normal for the period is 2.11 inches.

The extreme dryness last fall extended the fire danger into December, when 90-degree temperatures and strong Santa Ana winds helped the rapid spread of the Lilac fire in Fallbrook, which burned more than 4,000 acres and destroyed more than 150 structures.

El Niños tend to increase the odds for a wetter-than-normal winter in Southern California, but they do not have a spotless track record. Before the winter of 2015-16, long-range forecasters thought a powerful El Niño was a sure bet to produce drought-busting rains, but the winter ended up drier than normal.

The weather service’s Matthew Moreland said the forecasters are keeping their fingers crossed that El Niño will deliver.

“If El Niño kicks in, the hope is that we’d have more precipitation in the fall,” Moreland said. “If we get above-normal rainfall, or even near normal, it would help with fuel moisture.”

But even if El Niño doesn’t bring the hoped-for rains, it could help the fire situation in another way. It could help stave off the Santa Anas.

“That’s a piece of good news for San Diego,” Krohn said.

Ramping up

Cal Fire’s Sanchez said regardless of the weather or the conditions in the back country, the agency is in a “constant state of readiness.” Cal Fire is in the process of adding one person to each of the 32 fire engines in the San Diego Unit. Twenty have already been hired.